1. Field of the Invention
The present invention relates to an optical sheet and an LCD (liquid crystal display) apparatus using the same, and more particularly, to an LCD apparatus in which a bright-line is decreased, brightness are enhanced, and the optical sheet is prevented from shrinking.
2. Description of the Related Art
An LCD apparatus employs the principle that a molecular arrangement of liquid crystal is varied when voltage is applied to the liquid crystal. The varied molecular arrangement causes double refraction, optical rotatory power, optical scattering, etc., thereby forming visible images. In other words, the LCD device displays the images by controlling the intensity of light passing through the liquid crystal.
The LCD device needs a light source because the LCD device cannot emit light by itself. Therefore, a back light assembly is employed for lightening an LCD panel.
FIG. 1 is an exploded sectional view of the conventional LCD apparatus. As shown therein, the conventional LCD apparatus comprises an LCD panel 110 displaying the images thereon, and the back light assembly 200 provided behind the LCD panel 110 and lightening the LCD panel 110.
The back light assembly 200 comprises a lamp unit 130 used as the light source, a light guide plate 120 guiding the light from the lamp unit 130 uniformly, and a plurality of optical sheets 160 to enhance the brightness.
The lamp unit 130 is provided in a lateral side of the light guide plate 120, and the light emitted from the lamp unit 130 enters the light guide plate 120 through the lateral side of the light guide plate 120. Further, a lamp reflector 140 is provided in the lateral side of the light guide plate 120, covering the lamp unit 130. The lamp reflector 140 reflects the light arriving at the lamp reflector 140 into the light guide plate 120, thereby decreasing light loss. Here, a CCFL (cold cathode fluorescent lamp) is generally used as the lamp unit 130.
The light guide plate 120 guides the light from the lamp unit 130 toward the LCD panel 110 uniformly. On the rear surface of the light guide plate 120 is formed a fine pattern such as a dotted pattern, etc., thereby reflecting the light toward the LCD panel 110.
Behind the light guide plate 120 is generally placed a reflector 150 to reflect the light arriving at the reflector 150 into the light guide plate 120, thereby decreasing the light loss.
The plurality of optical sheets 160 is placed between the light guide plate 120 and the LCD panel 110, to thereby decrease a bright-line and enhance the brightness. The optical sheets 160 generally include a diffusing sheet 161, a first prism sheet 162, a second prism sheet 163, etc.
The diffusing sheet 161 diffuses the light guided by the light guide plate 120, thereby uniformly transmitting the light to the LCD panel 110 without uneven brightness.
The first prism sheet 162 and the second prism sheet 163 are sequentially placed in front of the diffusing sheet 161 and are respectively formed with a plurality of prism teeth 162a and 163a at intervals of a predetermined pitch. The first and second prism sheets 162 and 163 condense the light diffused by the diffusing sheet 161 onto the LCD panel 110, thereby enhancing the brightness. Here, the prism teeth 162a of the first prism 162 are perpendicularly to the prism teeth 163a of the second prism 163, thereby condensing the light lengthwise and crosswise. Generally, the first and second prism sheets 162 and 163 are respectively manufactured by coating base layers 162b and 163b made of PET (polyethylene terephthalate) with acrylic resin and then forming the prism teeth 162a and 163a on the acrylic resin.
On the other hand, in the conventional LCD apparatus, a reflective polarizing film (DBEF: dual brightness enhancement film) 164 can be added between the second prism sheet 163 and the LCD panel 110 in order to more enhance the brightness. The reflective polarizing film 164 allows the light from the light guide plate 120 to be repeatedly reflected and transmitted, thereby enhancing the brightness. For example, P-wave components of the light from the lamp unit 130 are transmitted through the reflective polarizing film 164 to the LCD panel 110, and S-wave components thereof are reflected from the reflective polarizing film 164 toward the light guide plate 120. Here, the S-wave components of the light reflected from the reflective polarizing film 164 are reflected again from the diffusing sheet 161, the light guide plate 120 or the reflector 150. On this way, the reflected S-wave components are changed into the P-wave components and then transmitted through the reflective polarizing film 164, thereby decreasing the light loss.
However, the conventional LCD apparatus has following problems.
First, because the prism sheet 162 and 163 is manufactured by coating only one surface of the base layer 162b and 163b made of the PET with acrylic resin, the prism sheet 162 and 163 can be easily shrunk, and moire pattern can easily occur by interference between the prism sheet 162 and 163 and the LCD panel 110.
Second, the prism sheet 162 and 163 causes the bright-line at the lateral sides, through which light enters, of the light guide plate 120. Particularly, if the LCD apparatus has a reverse prism sheet structure, the bright-line can all the more occur at the lateral sides of the light guide plate 120.
Third, the reflective polarizing film 164 enhances the brightness, but is relatively expensive, thereby increasing production cost of the LCD apparatus.